The present invention relates to a method of operating an adsorption refrigeration system.
Adsorption refrigeration systems are well known in the field of refrigeration and particularly cryogenics, for providing very low temperatures in a region such as a chamber. Adsorption refrigeration systems operate by the provision of an amount of liquid coolant within a chamber to be cooled. This is placed in gaseous communication with an amount of an adsorbing xe2x80x9csorptionxe2x80x9d material such as charcoal, the entire system being closed such that the amount of coolant within the system remains constant.
Typically the coolant in liquid form is obtained by condensation of gaseous coolant in contact with the cold walls of a member pre-cooled by an external source. This is performed in a conventional adsorption refrigeration system by the use of a xe2x80x9c1K potxe2x80x9d.
A second, alternative method of obtaining liquid coolant uses an expansion process, in which gaseous coolant is decompressed from a high pressure under adiabatic conditions. This decompression causes liquefaction of the gas thereby generating the liquid coolant. The provision of liquid coolant by this method has been used in an experimental adsorption refrigeration system. However, only a short hold time was achieved in this case with respect to commercial systems having a 1K pot.
The adsorption material of the system is arranged to adsorb the gas above the liquid coolant such that further evaporation of the liquid occurs due to the corresponding reduction in the pressure. The latent heat of evaporation causes a reduction in the temperature of the system.
However, one problem with such adsorption systems is that the sorption material itself can only adsorb a finite amount of gas for a given pressure. Such devices are therefore effectively of the single-shot type, with typical commercial systems being operable for a number of hours, although this is dependent upon the sorption capacity of the sorption material.
Adsorption systems are advantageous in that they are relatively simple devices which can be recharged by simply heating the sorption material so as to cause desorption of the gas-coolant and return it to the gaseous phase. Upon sufficient subsequent cooling, the adsorption material can be reused. As the system is enclosed, there is no loss of coolant and there are no moving parts. This is beneficial in that low temperature experiments can be performed at low levels of vibration for many hours.
The primary problem with conventional adsorption refrigeration systems is the single-shot nature of their operation and therefore, for experiments that require longer periods of operation, conventional adsorption pumps are impractical.
Conventional systems having a 1K pot tend to be expensive due to the provision of the 1K pot and its support systems. In addition, the use of a 1K pot does also produce some vibrations, which it is desirable to eliminate.
In the alternative experimental system described above the expansion of the gaseous coolant was used to generate the liquid phase coolant rather than using a 1K pot. Thus, although the 1K pot was replaced by using the expansion effect, unfortunately the hold time at operational temperatures was significantly reduced.
There is therefore a strong market demand to improve the operational hold time of such devices along with the elimination of vibrations.
In accordance with the present invention we provide a method of operating an adsorption refrigeration system, the system comprising an adsorption pump which, in use, is arranged in communication with a chamber containing liquid and gaseous coolant, the method comprising:
i) expanding the gaseous coolant into an auxiliary volume member so as to cause the removal of part of the gaseous coolant from the chamber, thereby reducing the temperature and pressure of the gaseous coolant in the chamber; and
ii) operating the adsorption pump so as to further cool the chamber by causing the evaporation of the coolant liquid within the chamber.
We have therefore adopted a new approach in improving the performance of single-shot adsorption devices by the use of an expansion process in which gaseous coolant is expanded into an auxiliary volume member. When a gas is allowed to increase its volume under certain conditions (for example adiabatic conditions), the reduction in pressure produced causes a corresponding reduction in the temperature of the gas. We have realised that, in this way, the expansion effect can be used to greater effect in association with the evaporation cooling of a known adsorption system.
This significantly increases the time at which the chamber can be maintained at operational temperatures, which are suitable for activities such as experimentation. Whereas the known experimental expansion process obtained only a short hold time of a few hours at operational temperatures, with the present invention, hold times of 50 hours or more are realisable without the expense and vibration problems associated with conventional systems having a 1K pot. This is due to the use of an auxiliary volume member which allows for a much improved expansion effect. An increased volume of liquid coolant can therefore be generated and a lower starting temperature is also attainable before the conventional evaporation cooling step begins.
The expansion stage of the method may either be performed once in a single step, or in a number of separate sub-steps in a multistage process with, for example, consecutive pressure reductions, in order to further reduce the temperature in the chamber prior to performing step (ii). Typically in this case step (i) further comprises expanding the gaseous coolant separately into a number of additional auxiliary volume members. Prior to the expansion step (i), the quantity of gaseous coolant supplied to the adsorption refrigeration system is preferably in excess of the saturation limit of the adsorbent material within the adsorption pump when operating under normal working conditions.
Prior to step (i), the coolant may be provided either as a gas or a liquid from any suitable source. However, preferably the coolant is provided as a gas from an auxiliary volume member used in step (i).
The auxiliary volume is preferably a static volume provided by a storage reservoir or a second adsorption pump. The auxiliary volume member may be arranged to have a constant geometrical volume or a variable volume. The use of a variable volume member allows the pressure within the chamber to be controlled and therefore the degree of cooling can be controlled accordingly.
In either case, the expansion of the gaseous coolant may be effected by allowing the gas to expand into the auxiliary volume member. This is generally performed using a controllable valve.
Typically the capacity of the auxiliary volume member is greater than the adsorption capacity of the adsorption pump and this ensures that the single-shot operational time is maximised.
Although the adsorption pump may be separated from the chamber using an appropriate valve such that steps (i) and (ii) of the method are separable, typically the adsorption pump remains in communication with the chamber during the steps of providing and/or expanding the cooling gas to and from the chamber respectively. The operational simplicity of the method is therefore improved.
In such a case, prior to step (i) the adsorption pump is cooled to the first temperature such that the adsorption material contained therein adsorbs the gaseous coolant so as to become substantially saturated for pressures higher than the ultimate pressure obtained at the lowest temperature. Preferably the adsorption pump is then disconnected from the storage vessel and heated so as to desorb gaseous coolant and thereby increase the gas pressure in the chamber. This increase in pressure may be in addition to a positive pressure of gas provided when the gaseous coolant is initially supplied to the chamber prior to step (i).
Typically the adsorption pump is also heated during step (i) so as to maximise the effect of the first stage expansion to the auxiliary volume member.
Following the expansion step (i), the adsorption system is typically isolated from the auxiliary volume member during the subsequent step (ii) so as to maximise the single-shot operational time.
Advantageously in some cases the expansion effect may also be used when the adsorption pump system is no longer in communication with the auxiliary volume member. This may be achieved by cooling the adsorption pump prior to step (ii), thereby further reducing the pressure of the gaseous coolant within the chamber. This effectively expands the gaseous coolant further and causes further cooling in a second expansion step. It was an analogous step of this kind, using only the internal volume of an adsorption pump system, that was used in the known experimental expansion cooling method (described earlier) to generate all of the liquid coolant.
In contrast, the present invention preferably uses this additional expansion process and/or that of step (i) to cause the partial liquefaction of the gaseous coolant.
The method can be used with many known coolants such as helium-4, nitrogen, neon or hydrogen although it is particularly suitable for use with helium-3 as this provides the capability of attaining the lowest temperatures for experimental purposes.